U.S. patent application number 12/956560 was filed with the patent office on 2012-05-31 for neutron porosity logging tool using microstructured neutron detectors.
This patent application is currently assigned to SONDEX LIMITED. Invention is credited to Helene Claire CLIMENT.
Application Number | 20120132819 12/956560 |
Document ID | / |
Family ID | 45316048 |
Filed Date | 2012-05-31 |
United States Patent
Application |
20120132819 |
Kind Code |
A1 |
CLIMENT; Helene Claire |
May 31, 2012 |
Neutron Porosity Logging Tool Using Microstructured Neutron
Detectors
Abstract
A neutron porosity measurement device uses semiconductor
detectors located at different distances from a cavity configured
to accommodate a neutron source. Each of the semiconductor
detectors includes (i) a semiconductor substrate doped to form a pn
junction, and having microstructures of neutron reactive material
formed to extend from a first surface inside the semiconductor
substrate, and (ii) electrodes, one of which is in contact with the
first surface of the semiconductor substrate and another one of
which is in contact with a second surface of the semiconductor
substrate, the second surface being opposite to the first surface.
The electrodes are configured to acquire an electrical signal
occurring when a neutron is captured inside the semiconductor
substrate.
Inventors: |
CLIMENT; Helene Claire;
(Sugar Land, TX) |
Assignee: |
SONDEX LIMITED
Yately
GB
|
Family ID: |
45316048 |
Appl. No.: |
12/956560 |
Filed: |
November 30, 2010 |
Current U.S.
Class: |
250/370.05 ;
29/825 |
Current CPC
Class: |
Y10T 29/49117 20150115;
G01V 5/107 20130101; G01T 3/08 20130101 |
Class at
Publication: |
250/370.05 ;
29/825 |
International
Class: |
G01T 3/08 20060101
G01T003/08; H01R 43/00 20060101 H01R043/00 |
Claims
1. A neutron porosity measurement device, comprising: a cavity
configured to receive a neutron source that emits neutrons; a first
semiconductor detector located at a first distance from the cavity;
a second semiconductor detector located at a second distance larger
than the first distance from the cavity, wherein each of the first
and the second semiconductor detector includes a semiconductor
substrate doped to form a pn junction, and having microstructures
of neutron reactive material formed to extend from a first surface
inside the semiconductor substrate, and electrodes, one of which is
in contact with the first surface of the semiconductor substrate
and another one of which is in contact with a second surface of the
semiconductor substrate, the second surface being opposite to the
first surface, the electrodes being configured to acquire an
electrical signal occurring when a neutron is captured inside the
semiconductor substrate; and an electronics block located between
the first semiconductor detector and the second semiconductor
detector and configured to receive the electrical signal from the
electrodes, wherein no high power source is included to provide an
electric field across the first semiconductor detector and/or to
the second semiconductor detector, a space between the first
semiconductor detector and the second semiconductor detector,
except for the electronics block, being filled with a neutron
absorber.
2-3. (canceled)
4. The neutron porosity measurement device of claim 1, wherein the
electronics block is configured to count a number of electrical
signal received from the first semiconductor detector and a number
of the electrical signal received from the second semiconductor
detector in a predetermined time interval.
5. The neutron porosity measurement device of claim 4, wherein the
electronics block is configured determine a ratio of the number of
the electrical signal received from the first semiconductor
detector and the number of the electrical signal received from the
second semiconductor detector in the predetermined time
interval.
6. The neutron porosity measurement device of claim 4, further
comprising at least one of: a memory configured to store data
including the number of electrical signal received from the first
semiconductor detector and the number of the electrical signal
received from the second semiconductor detector in the
predetermined time interval, for a sequence of intervals; and a
processing unit configured to calculate a porosity value based on a
ratio of the number of the electrical signal received from the
first semiconductor detector and the number of the electrical
signal received from the second semiconductor detector in the
predetermined time interval.
7. The neutron porosity measurement device of claim 1, wherein the
electronics block is configured to supply a low power potential
difference to the electrodes and the electrodes are configured to
apply an enhancing electric field across the semiconductor
substrate.
8. The neutron porosity measurement device of claim 1, wherein the
electronics block is configured to transmit data related to a
neutron porosity measurement to a remote device.
9. The neutron porosity measurement device of claim 1, wherein the
electronics block is configured to operate at temperatures up to at
least 250.degree. C.
10. The neutron porosity measurement device of claim 1, wherein the
semiconductor substrate includes trenches configured to receive the
neutron reactive material.
11. The neutron porosity measurement device of claim 1, wherein the
semiconductor substrate includes holes configured to receive
pillars of the neutron reactive material.
12. The neutron porosity measurement device of claim 1, wherein the
neutron reactive material comprises .sup.10B.
13. The neutron porosity measurement device of claim 1, wherein the
neutron reactive material comprises .sup.6Li.
14. The neutron porosity measurement device of claim 11, wherein
the neutron reactive material is .sup.6LiF.
15. The neutron porosity measurement device of claim 1, wherein a
thickness of the microstructures of neutron reactive material from
the first surface inside the semiconductor substrate is between 50
.mu.m and 200 .mu.m.
16. The neutron porosity measurement device of claim 1, wherein the
semiconductor substrate is silicon carbide.
17. The neutron porosity measurement device of claim 1, further
comprising: a chassis encapsulating the cavity, the first
semiconductor detector and the second semiconductor detector, which
are arranged coaxially.
18. A neutron porosity measurement tool, comprising: a neutron
source that emits neutrons; a first semiconductor detector located
at a first distance from the neutron source; a second semiconductor
detector located at a second distance larger than the first
distance from the neutron source; an electronics block configured
to receive electrical signals from the first semiconductor detector
and the second semiconductor detector; and a chassis configured to
accommodate the neutron source, the first semiconductor, the
electronics block and the second semiconductor detector, wherein
each of the first and the second semiconductor detector includes a
semiconductor substrate doped to form a pn junction, and having
microstructures of neutron reactive material formed to extend from
a first surface inside the semiconductor substrate, and electrodes,
one of which is in contact with the first surface of the
semiconductor substrate and another one of which is in contact with
a second surface of the semiconductor substrate, the second surface
being opposite to the first surface, the electrodes being
configured to acquire and transmit to the electronics block, an
electrical signal occurring when a neutron is captured in the
semiconductor substrate, wherein no high power source is included
to provide an electric field across the first semiconductor
detector and/or to the second semiconductor detector, a space
between the first semiconductor detector and the second
semiconductor detector, except for the electronics block, being
filled with a neutron absorber.
19. A method of manufacturing a neutron porosity measurement
device, comprising: mounting a first semiconductor detector, an
electronics block and a second semiconductor detector in this order
along a chassis, above a cavity of the chassis configured to
accommodate a neutron source; and connecting the electronics block
to pairs of electrodes of the first semiconductor detector and of
the second semiconductor detector, to acquire electrical signals
produced in the first semiconductor detector and in the second
semiconductor detector when a neutron is captured therein.
20. The method of claim 19, further comprising at least one of:
attaching the chassis to a drill line; filling a space between the
first semiconductor detector and the second semiconductor detector,
except for the electronics block, with a neutron absorber; and
mounting the neutron source inside the cavity.
21. The neutron porosity measurement device of claim 1, wherein the
neutron absorber is boron epoxy.
22. The neutron porosity measurement tool of claim 19, wherein the
neutron absorber is boron epoxy.
Description
BACKGROUND
[0001] 1. Technical Field
[0002] Embodiments of the subject matter disclosed herein generally
relate to methods and tools used to measure formation porosity in
oil and gas industry, more particularly, to methods and tools using
microstructured semiconductor neutron detectors.
[0003] 2. Discussion of the Background
[0004] In the oil and gas industry, formation porosity is measured
to identify oil and gas reserves. Although other techniques may be
employed to determine formation porosity (e.g., sonic and Nuclear
Magnetic Resonance), the porosity measurements using neutrons is
the most frequent.
[0005] Down-hole neutron-porosity tools may be wireline or logging
(or measuring) while drilling (LWD/MWD). The principal difference
between LWD and wireline tools is the service environment. LWD
tools operate during the drilling process and are subjected to the
high levels of vibration and shock generated by drilling through
rock. Wireline tools are conveyed in and out of the borehole on a
cable after drilling, and, therefore, do not experience shock and
vibration. In both cases, the tool operates at temperatures as high
as 175.degree. C. sometimes higher.
[0006] As illustrated in FIG. 1, down-hole porosity measurements
are performed using a neutron source 10 and two detectors or arrays
of detectors, a "near" neutron detector 20 and a "far" neutron
detector 30, which are located at different distances from the
neutron source 10. The neutron source 10 and the neutron detectors
20 and 30 are usually encapsulated in a chassis 40. The chassis 40
is lowered in a borehole 50 that penetrates a soil formation 60.
Some of the neutrons emitted by the neutron source 10 towards the
soil formation 60, loose energy (i.e., are "thermalized") and are
deflected towards the neutron detectors 20 and 30 due to collisions
or interactions with nuclei in the formation 60.
[0007] The detectors 20 and 30 detect some (depending on each
detector's efficiency) of these neutrons with lower (thermal)
energy. The ratio of the counting rates (i.e., number of detected
neutrons/time) in the two detectors 20 and 30 is directly related
to the porosity of the formation 60.
[0008] The probability of an interaction of a neutron and a nucleus
(i.e., a nuclear reaction) can be described by a cross-section of
the interaction (i.e., reaction). A detector's efficiency is
proportional with the probability of an interaction occurring when
a neutron enters the detector's volume. The neutron detectors are
built based on the large probability (i.e., cross-section) of a
thermal neutron being captured (i.e., interact/react) with three
nuclei: helium (.sup.3He), lithium (.sup.6Li) and boron (.sup.10B).
Other particles such as, the .alpha. particle (.sub.2.sup.4.alpha.)
and the proton (.sub.1.sup.1p) result from the reaction of the
thermal neutron with these elements. A calculable amount of energy
(Q) is emitted as a result of the neutron capture reaction. This
emitted energy may be kinetic energy of the resulting particles or
gamma rays. The energy is dissipated by ionization, that is,
formation of pairs of electron and positively charged particle.
These pairs can be collected, for example, in an electrical field,
and, thus, generate a signal recognizable as a signature of the
neutron capture reaction. The larger is the emitted energy, the
larger is the amplitude of the signature signal.
[0009] Some other particles (e.g., gamma rays) besides the targeted
neutrons may cross the detector simultaneously. A good detector
should exhibit characteristics that would allow discrimination
between capture of a thermal neutron and other untargeted nuclear
reactions that may occur. To facilitate discrimination between a
neutron capture reaction and a gamma ray, the energy emitted in the
neutron capture reaction (Q) should be as high as possible.
[0010] The three most common neutron capture reactions used for
neutron detection are illustrated in Table 1:
TABLE-US-00001 TABLE 1 Thermal neutron cross section Name Reaction
Q (MeV) (barns) .sup.10B(n, .alpha.) .sub.5.sup.10B +
.sub.0.sup.1n.fwdarw..sub.3.sup.7Li + .sub.2.sup.4.alpha. Ground
2.792 3840 Excited 2.31 .sup.6Li(n, .alpha.) .sub.3.sup.6Li +
.sub.0.sup.1n.fwdarw..sub.1.sup.3H + .sub.2.sup.4.alpha. 4.78 940
.sup.3He(n, p) .sub.2.sup.3He + .sub.0.sup.1n.fwdarw..sub.1.sup.3H
+ .sub.1.sup.1p 0.764 5330
[0011] In the above table, relative to the .sup.10B(n, .alpha.)
reaction "Ground" means that the resulting .sub.3.sup.7Li is in a
ground state and "Excited" means that the resulting .sub.3.sup.7Li
is in the first excited state.
[0012] Traditionally, detectors based on .sup.3He(n, p) reaction
have been used in neutron porosity measurements performed in the
oil and gas industry, due to their relatively low cost, ruggedness,
good detection efficiency, and insensitivity to gamma rays (i.e.,
the cross section for an interaction of the gamma ray with .sup.3He
is very small). The detection efficiency of these .sup.3He based
detectors can be improved by using higher pressures of the .sup.3He
gas, but the use of higher pressures results in increasing the cost
of the detectors and of the high voltage required to operate them,
which adversely affects the associated detector electronics.
Additionally, the critical worldwide shortage of .sup.3He makes it
necessary to develop alternate neutron detectors for neutron
porosity measurements in the oil and gas industry.
[0013] Lithium-glass scintillation detectors are currently used in
some logging tools. The detection efficiency of the detectors based
on .sup.6Li(n, .alpha.) reaction depends on the amount of .sup.6Li
in the detector material. A common lithium-glass used for down-hole
logging is GS20, which has an isotopic ratio of 95% .sup.6Li and a
total lithium composition of 6.6%. Although the cross section for
an interaction of the gamma ray with .sup.6Li is significant, the
large amount of energy (Q) resulting from the .sup.6Li(n, .alpha.)
reaction enables a reasonable discrimination from reactions induced
by gamma rays. However, the poor energy resolution of lithium-glass
detectors at room temperature diminishes further at temperatures as
low as 150.degree. C., rendering their use limited to relatively
shallow wells. In the lithium-glass scintillation detectors, the
lithium-glass is coupled to a photomultiplier tube (PMT) that
introduces electronic noise at elevated temperatures and is
mechanically fragile.
[0014] Accordingly, it would be desirable to provide neutron
detectors having a good detection efficiency (i.e., large cross
section for neutron capture), good discrimination relative to gamma
rays, and can be used in the logging shock and vibration
environment (e.g., during drilling) and at high temperatures (e.g.,
over 175.degree. C.).
SUMMARY
[0015] According to one exemplary embodiment, a neutron porosity
measurement device includes a cavity configured to receive a
neutron source that emits neutrons, a first semiconductor detector
located at a first distance from the cavity, and a second
semiconductor detector located at a second distance larger than the
first distance from the cavity. Each of the first and the second
semiconductor detector includes a semiconductor substrate doped to
form a pn junction, and having microstructures of neutron reactive
material formed to extend from a first surface inside the
semiconductor substrate, and electrodes, one of which is in contact
with the first surface of the semiconductor substrate and another
one of which is in contact with a second surface of the
semiconductor substrate, the second surface being opposite to the
first surface, the electrodes being configured to acquire an
electrical signal occurring when a neutron is captured inside the
semiconductor substrate.
[0016] According to one exemplary embodiment, a neutron porosity
measurement tool includes (i) a neutron source that emits neutrons,
(ii) a first semiconductor detector located at a first distance
from the neutron source, (iii) a second semiconductor detector
located at a second distance larger than the first distance from
the neutron source, (iv) an electronics block configured to receive
electrical signals from the first semiconductor detector and from
the second semiconductor detector, and (v) a chassis configured to
accommodate the neutron source, the first semiconductor, the
electronics block and the second semiconductor detector. Each of
the first and the second semiconductor detector includes a
semiconductor substrate doped to form a pn junction, and having
microstructures of neutron reactive material formed to extend from
a first surface inside the semiconductor substrate, and electrodes,
one of which is in contact with the first surface of the
semiconductor substrate and another one of which is in contact with
a second surface of the semiconductor substrate, the second surface
being opposite to the first surface, the electrodes being
configured to acquire and transmit to the electronics block, an
electrical signal occurring when a neutron is captured in the
semiconductor substrate.
[0017] According to another exemplary embodiment, a method of
manufacturing a neutron porosity measurement device includes
mounting a first semiconductor detector, an electronics block and a
second semiconductor detector in this order along a chassis, above
a cavity of the chassis configured to accommodate a neutron source.
The method further includes connecting the electronics block to
pairs of electrodes of the first semiconductor detector and of the
second semiconductor detector, to acquire electrical signals
produced in the first semiconductor detector and in the second
semiconductor detector when a neutron is captured therein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate one or more
embodiments and, together with the description, explain these
embodiments. In the drawings:
[0019] FIG. 1 is a schematic diagram of a neutron detection
tool;
[0020] FIG. 2 is a schematic diagram of a neutron semiconductor
detector according to an exemplary embodiment;
[0021] FIG. 3 is a schematic diagram of a neutron semiconductor
detector according to another exemplary embodiment;
[0022] FIG. 4 is an illustration of a neutron detection in a
neutron semiconductor detector according to an exemplary
embodiment;
[0023] FIG. 5 is a schematic diagram of a neutron porosity
measurement device using neutron semiconductor detectors according
to an exemplary embodiment;
[0024] FIG. 6 is a graph illustrating the detection efficiency
dependence of the thickness of the reactive material in the
semiconductor material, according to various embodiments;
[0025] FIG. 7 is a graph illustrating a ratio of counting rates
relative to porosity of the soil formation, according to exemplary
embodiments; and
[0026] FIG. 8 is a flow chart of a method of manufacturing a
neutron porosity measurement device using semiconductor based
neutron detectors according to an exemplary embodiment.
DETAILED DESCRIPTION
[0027] The following description of the exemplary embodiments
refers to the accompanying drawings. The same reference numbers in
different drawings identify the same or similar elements. The
following detailed description does not limit the invention.
Instead, the scope of the invention is defined by the appended
claims. The following embodiments are discussed, for simplicity,
with regard to the terminology and structure of neutron detection
tools used for measuring the porosity of soil formations in oil and
gas industry. However, the embodiments to be discussed next are not
limited to these systems, but may be applied to other systems that
require neutron detection in the context of scarcity of
.sup.3He.
[0028] Reference throughout the specification to "one embodiment"
or "an embodiment" means that a particular feature, structure, or
characteristic described in connection with an embodiment is
included in at least one embodiment of the subject matter
disclosed. Thus, the appearance of the phrases "in one embodiment"
or "in an embodiment" in various places throughout the
specification is not necessarily referring to the same embodiment.
Further, the particular features, structures or characteristics may
be combined in any suitable manner in one or more embodiments.
[0029] Neutron semiconductor detectors consist of a pn junction
diode made of semiconductor microstructured with an etched pattern
and filled with neutron reactive material. The pn junction is
manufactured by doping the semiconductor substrate (1) with
impurities having more than four electrons on its outer electron
shell, on one side of the semiconductor substrate, and (2) with
impurities having less than four electrons on its outer electron
shell or on one side of the semiconductor substrate. Doping the
semiconductor substrate in this manner determines occurrence of an
electric field inside the semiconductor structure. The electric
field favors a movement of free positively charged particles
towards one surface of the semiconductor substrate and a movement
of free negatively charged particles towards an opposite surface of
the semiconductor. Thus, one advantage of the neutron semiconductor
detector is that, in contrast with the .sup.3He and Lithium-glass
detectors, no applied electric field is necessary for detecting a
signal due to collecting the free charged particles generated after
a neutron capture, at the electrodes.
[0030] FIG. 2 illustrates a neutron semiconductor detector 100 made
of a semiconductor 110 (p+ and n+ illustrate the presence of
impurities in the semiconductor 110) with thin film microstructures
of neutron reactive material 120 formed to extend from a first
surface 115. The neutron reactive material 120 may include .sup.10B
or .sup.6Li (e.g., LiF). The semiconductor 110 may be silicon
carbide (SiC). In FIG. 2, the neutron reactive material 120 is
inserted in trenches inside the semiconductor material 110. In a
part of the semiconductor 110 where the neutron reactive material
is inserted in trenches, a neutron reactive material concentration
by volume may be about 50% of the total volume. In an alternative
embodiment illustrated in FIG. 3, a neutron semiconductor detector
101 is inserted in holes, the neutron reactive material 120 forming
pillars inside the semiconductor material 110. In a part of the
semiconductor 110 where the neutron reactive material is inserted
in the holes, a neutron reactive material concentration by volume
may be about 12% of the total volume. In trenches or pillars, the
neutron reactive material 120 having a thickness h penetrates
inside the semiconductor 110. The thickness h ranges from tens of
micrometers up to a few hundreds of micrometers.
[0031] FIG. 4 illustrates the detection of a neutron in a neutron
semiconductor detector 150 (e.g., 100 in FIG. 2 or 101 in FIG. 3).
An incoming thermal neutron 152 interacts with a .sup.6Li nucleus
154 inside the neutron reactive material 120. The .alpha.-particle
156 and the recoiled nucleus 154 resulting from the interaction
generates pairs 158 of free electrons and missing electron holes
(behaving like free positively charged particles) in the
semiconductor 110. The electrons and the holes migrate to the top
or the bottom of the structure due to the electric field in the
semiconductor pn junction. The electric field in the semiconductor
due to the doping may be amplified by an electric field applied via
electrodes 160 and 170. The electrodes 160 and 170 may be made of
gold. The electrodes 160 and 170 allow acquiring and transmitting
an electrical signal due to the free charged particles formed after
a neutron is captured.
[0032] Neutron semiconductor detectors as described above are used
in a neutron porosity measurement device 200 illustrated in FIG. 5.
A chassis 205 includes at a cavity 208 in a lower part of the
chassis 205. The cavity 208 is configured to receive a fast neutron
source 210. Due to the potential harmful effects of human exposure
to neutron radiation, chemical neutron sources are usually stored
in safe containers when not used and mounted only shortly before
the tool is lowered in the borehole for measurement. Here, "up" and
"down", "top" and "bottom", "above" and "beneath" correspond to the
illustration in FIG. 5, which illustrates the orientation of the
chassis 205 as lowered in the borehole, in z direction. The neutron
source 210 may be a chemical source such as AmBe or an electrical
neutron generator. The chassis 205 may be made of stainless
steel.
[0033] The neutrons emitted by the neutron source are thermalized
or slowed down in the formation and in the borehole. The slowdown
and scattering of the neutrons towards the detectors in the
formation is the effect that makes possible evaluating the
formation porosity. The slowdown and scattering occurring in the
borehole is an undesired effect, which makes it necessary to apply
a correction, known as the borehole correction to the measurement
results. Other corrections applied while processing the porosity
measurement results include (but are not limited to): mud weight,
temperature, pressure, standoff from the borehole wall, etc.
[0034] A "near" semiconductor detector 220 is located in the
chassis 205, above the neutron source 210. For example, the near
semiconductor detector 220 may be located around 5'' away from the
neutron source 210. A "far" semiconductor detector 230 is located
in the chassis 205, above the neutron source 210 and the "near"
semiconductor detector 220. For example, the far semiconductor
detector 230 may be located around 15'' away from the neutron
source 210. The neutron source 210 and the semiconductor detectors
220 and 230 may be mounted coaxially. An average energy of neutrons
emitted by the neutron source is substantially larger than an
average energy of neutrons detected in the semiconductor detectors
220 and 230. Fast neutrons, having for example energies larger than
1 MeV, are less likely to interact with nuclei due to neutrons'
high speeds. Thermal neutrons with energies less than 1 eV are more
likely captured by nuclei. Therefore, an average energy of neutrons
emitted by the neutron source is substantially larger than an
average energy of neutrons detected in the semiconductors.
[0035] Between the near semiconductor detector 220 and the far
semiconductor detector 230 in the chassis 205 may be located an
electronics block 240. The electronics block 240 may include a
measurement data processing unit 243 configured to collect and
process data (e.g., electrical signals) from the semiconductor
detectors 220 and 230. The data processing unit 243 may be
configured to count a number of electrical signals received from
the semiconductor detector 220 and a number of electrical signals
received from the semiconductor detector 230, during a
predetermined time interval, and to perform a number of corrections
before estimating the formation porosity based on a ratio of these
numbers. Thus, the electronics block 240 may determine and compare
counting rates for each of the neutron semiconductor detectors 220
and 230. In an alternative embodiment, the electronics block 240
may include a memory configured to store data related to the
porosity measurement, in order to retrieve and process the data
after the device is brought up at the surface.
[0036] The electronics block 240 may also be configured to transmit
data and or results related to the porosity measurement to a remote
device via a wire 245 or wirelessly. Although the wire 245 is
illustrated outside the chassis 205, the wire may be contained
inside the chassis 205 and may extend along a line (cable) used for
lowering the tool inside the borehole. Further, since the neutron
semiconducting detectors operate reliably at temperatures up to
250.degree. C., the electronics block 240 may be designed and built
to operate at the same temperatures. If the neutron porosity
measurement device 200 is used while drilling (i.e., Logging While
Drilling LWD, or Measuring While Drilling MWD), porosity
measurement related data may be sent at the surface through the mud
(in a real-time mode) and/or recorded in a data storage device (in
a recording mode) to be recovered and processed after the device is
brought back to the surface.
[0037] Conventional porosity logging tools using .sup.3He or
Lithium-glass detectors require a high voltage power supply to be
able to collect a signal when a neutron is captured. The high
voltage power supply takes up a lot of space inside the
conventional porosity logging tool. In case of the neutron porosity
measurement device 200, no power supply is necessary due to the pn
junction's electric field (although an enhancing electric field may
be applied without a high power requirement). Since the neutron
semiconductors detectors do not require a (high) power supply, the
electronics block is smaller compared to that of a helium tube or
the photomultiplier required with lithium-glass. If a chassis used
for a conventional tool is used with neutron semiconductor
detectors, the freed space may be filled with a neutron absorber
(boron epoxy, for example) to better shield the neutron detectors
from the neutron source, and, thus, to lower the number of detected
neutrons that do not travel through the formation. In other words,
more shielding results in reducing noise of the measurement.
Alternatively, the electronics block being smaller allows the
chassis to be smaller than the chassis used for a conventional
tool.
[0038] The detection efficiency of different types of detectors has
been compared using simulations using the MCNP code. The MCNP code
is a Monte Carlo N-particle Transport Code software developed for
simulating nuclear processes. The shape of the detectors used in
the simulation are based on exiting helium tube and lithium-glass
detectors, all the compared detectors having a cylindrical shape of
0.5'' diameter and 1'' length and being placed inside a chassis
having an outer diameter of 4'' and a length of 48''. The sizes of
the detectors of the exemplary embodiments may vary from these
numbers and may be adjusted through modeling to provide a neutron
porosity device capable to provide optimal performance in the
context for which it is designed. In these simulations, the
surrounding formation was considered for a depth of 71'' (outer
diameter) and a length of 48''. The characteristics of the
materials used in this simulation are summarized in Table 2. These
materials and numbers are exemplary and not intended to limit the
embodiments.
TABLE-US-00002 TABLE 2 Detector Material Helium tube 10 atm.
Pressure .sup.3He gas, 0.00134 g/cm.sup.3 Lithium-glass KG2,
isotopic ratio 95%, Total Lithium 7.5%, 2.42 g/cm.sup.3 SiC.sup.10B
pillar 87.44% SiC, 12.56% .sup.10B, 2.6736 g/cm.sup.3 SiC.sup.10B
trench 50% SiC, 50% .sup.10B, 2.8930 g/cm.sup.3 SiC.sup.6Li pillar
87.44 SiC, 12.56% .sup.6Li, 0.8636 g/cm.sup.3 SiC.sup.6Li trench
50% SiC, 50% .sup.6Li, 1.8580 g/cm.sup.3 Chassis Stainless
steel--15/5, 7.850 g/cm.sup.3 Formation Limestone 15.6 pu filled
with fresh water, 2.1398 g/cm.sup.3
[0039] The efficiency of the simulated detectors is determined as
the ratio between a captured neutron in the detector and the total
number of neutrons entering the detectors. The trenches and pillars
of neutron reactive material of the neutron semiconductor detectors
are 50 .mu.m deep. The results are summarized in Table 3.
TABLE-US-00003 TABLE 3 Detector Efficiency % Relative efficiency
Helium tube 2.51 Lithium-glass 6.67 2.66 SiC.sup.10B pillar 4.34
1.73 SiC.sup.10B trench 3.26 1.30 SiC.sup.6Li pillar 0.9 0.36
SiC.sup.6Li trench 0.54 0.22
[0040] The simulations revealed that neutron semiconductor
detectors with .sup.10B as reactive material have a higher
efficiency than the ones with .sup.6Li as reactive material for the
same dimensions which is expected as the cross-section of .sup.10B
is larger than the one of .sup.6Li.
[0041] Further simulations for pillars of neutron reactive material
having various depths in the semiconductor material, i.e., between
50 .mu.m and 200 .mu.m for a step of 50 .mu.m, revealed that (i) at
200 .mu.m depth of the pillars, the efficiency of the SiC.sup.6Li
detectors becomes comparable with the helium tube efficiency, and
for depths of the pillars over 130 .mu.m the SiC.sup.10B detectors
efficiency becomes larger than the lithium-glass efficiency. The
results are summarized in FIG. 6 in which the x-axis is the
pillar's thickness in .mu.m, on y-axis is the detection efficiency
in %, line 250 corresponds to the SiC.sup.6Li detector, line 260
corresponds to the helium tube, line 270 corresponds to the
SiC.sup.10B detector and line 280 corresponds to the lithium-glass
detector. The trench configuration for both SiC.sup.10B and
SiC.sup.6Li follow a similar trend since the reactive material
concentration increases with its thickness.
[0042] The simulations also allowed an evaluation of the ratio of
the counting rates of the near and far detectors for the
SiC.sup.10B detector, the SiC.sup.6Li detector, and the helium tube
as functions of the porosity of the formation. In FIG. 7, the
x-axis is the porosity in percentage, and the y-axis is the ratio
of the counting rates for the different neutron detectors. Line 300
in FIG. 7 corresponds to the SiC.sup.10B detector in pillar
configuration, line 310 to the SiC.sup.6Li detector in pillar
configuration, and line 320 to the helium tube. The slope of the
curves being larger for the neutron semiconductor detectors than
for the helium tube over 20% porosity means that the sensitivity of
the porosity measurement is larger when using the neutron
semiconductor detectors than when using the helium tube.
[0043] These simulation results have confirmed that the neutron
semiconductor detectors can be built to match and exceed the
detection performance (efficiency and sensitivity) of currently
used detectors (with .sup.3He and lithium-glass). Additionally, the
neutron semiconductor detectors can operate reliably at
temperatures up to 250.degree. C. and higher.
[0044] A flow chart of a method 400 of manufacturing a neutron
porosity measurement device is illustrated in FIG. 8. The method
400 includes mounting a neutron source, a first semiconductor
detector, an electronics block and a second semiconductor detector
in this order along a chassis, at S410. The method 400 further
includes connecting the electronics block to pairs of electrodes of
the semiconductor detectors, to provide a potential difference to
each pair of electrodes and to acquire electrical signals produced
in the semiconductor detectors when a neutron is captured therein,
at S420. The method 400 may further include, after S410 and S420,
attaching the chassis to a drill line.
[0045] The disclosed exemplary embodiments provide devices and
methods related to porosity measurements using neutron
semiconductor detectors. It should be understood that this
description is not intended to limit the invention. On the
contrary, the exemplary embodiments are intended to cover
alternatives, modifications and equivalents, which are included in
the spirit and scope of the invention as defined by the appended
claims. Further, in the detailed description of the exemplary
embodiments, numerous specific details are set forth in order to
provide a comprehensive understanding of the claimed invention.
However, one skilled in the art would understand that various
embodiments may be practiced without such specific details.
[0046] Although the features and elements of the present exemplary
embodiments are described in the embodiments in particular
combinations, each feature or element can be used alone without the
other features and elements of the embodiments or in various
combinations with or without other features and elements disclosed
herein.
[0047] This written description uses examples of the subject matter
disclosed to enable any person skilled in the art to practice the
same, including making and using any devices or systems and
performing any incorporated methods. The patentable scope of the
subject matter is defined by the claims, and may include other
examples that occur to those skilled in the art. Such other
examples are intended to be within the scope of the claims.
* * * * *